The present invention relates to a radiating h-f transmission system, including a coaxial high-frequency cable.
A cable of the type to which the invention pertains is comprised of an inner conductor, an outer conductor, and a dielectric spacer which is interposed between the conductors. The outer conductor is provided with openings through which the cable radiates. These cables are used, for example, for transmitting h-f signals from a stationary source to a movable vehicle, or vice versa. A particular need exists, for example, for continuing communication with a vehicle as it passes through a tunnel. In the case of rail vehicles, these cables are laid along the rails, e.g., on the ties, or next to the ties and/or support posts, or at a tunnel wall.
German printed patent application No. 10 44 199 suggests a radiating cable in which the outer conductor has a continuous, axis-parallel slot so that the outer conductor does not fully envelope the inner conductor. The slot is rather wide, covering about 100.degree. to 120.degree.. This width is deemed to be too wide as it attenuates transmission through the cable, particularly when laid upon a moist ground. Aside from lossiness, the large opening invites ingress and penetration of moisture into the dielectric spacer material, attenuating the transmitted signal further.
German printed patent application No. 16 90 138 discloses such a cable, in which the radiating opening is restricted to spaced, obliquely oriented slots. The direction of extension and orientation of these slots varies, resulting in a zigzag pattern. The purpose of this arrangement is to suppress the axial component of the emanating electromagnetic field while the radial component is enhanced resulting in a more uniform signal strength at the mobile receiver. Unfortunately, the sum total of all of the openings is still a rather large area, offering basically the same disadvantages mentioned above concerning lossiness and/or moisture penetration.
German printed patent application No. 28 11 904 discloses a transmission system using a h-f cable having a physically closed outer conductor; in other words, the cable as such is a conventional, nonradiating one. Certain coaxial components are interspersed in the line, having radiating openings. This system is based upon the principle that radiation leaving such an opening is in parts transmitted as a surface wave on and along the outside of the closed, coaxial cable portion leading to the next radiating component. This system, however, poses the problem that the joints must have the same wave impedance as the h-f cable in order to avoid reflections. Also, the installation of this system requires highly skilled labor.
German printed patent application No. 24 03 646 discloses a radiating h-f cable in which openings are provided at a uniform, relatively short, spacing. The spacing of the openings is about equal to the outer diameter of the outer conductor and is, thus, considerably smaller than the wavelength of the signal to be transmitted. In view of the relatively large number of openings, the open area is still quite large; and again, one will encounter the disadvantages of water ingress, mentioned above.